This invention relates to the field of integrated circuits and particularly to probes used for testing high speed integrated circuits.
High speed integrated circuits have some very special needs which stem from the fact that they are generally more vulnerable to noise and that some of the sources of noise are enhanced under high speed conditions. One of these sources of noise is "ringing" which can originate in the probes used to test integrated circuits prior to packaging.
During the fabrication of integrated circuits, it is necessary to know which devices are worth packaging before going to the trouble and expense of doing so. Therefore, high speed testing must be done on the wafer. To perform high speed testing on a circuit while probing a wafer calls for a special probe set in order to prevent problems due to ringing.
The prime consideration in probing systems is that a mechanical system must be assembled which will simultaneously contact many pads in a very confined space which may be a millimeter on each side. The contact pads may be only 50 .mu.m on a side. It is virtually impossible to bring controlled impedance lines into such an array, and the usual probe system consists of tungsten needle probes all converging into the area of the circuit.
Each needle probe is modeled as a high impedance line section 2 cm long with one side terminated in a 50 ohm termination and the other open circuit at the high impedance input of the digital integrated circuit. This is modeled by a simple equivalent circuit, taking the needle probe to be an air dielectric transmission line section having a characteristic impedance of 250 ohms. For a 500 M Bit signal, which is square, i.e. does have Fourier components up to 2 GHz, the simulation calculation shows that the resultant signal at the circuit end of the probe is considerably distorted. It looks more like a 1.5 GB signal to which a circuit with a 120 p sec switching rate would respond quite readily. Such effects can create a considerable error and shows the need for reliable test probes devoid of "ringing problems".